This application is based on application Nos. 97-65103 and 98-1855 filed in Korean Industrial Property Office on Dec. 1, 1997 and Jan. 22, 1998 respectively, the contents of which are incorporated hereinto by reference.
The present invention relates to a shadow mask for a cathode ray tube (CRT) and a method of manufacturing the same and, more particularly, to a method of manufacturing a CRT shadow mask having improved tensional strength and elongation.
A CRT shadow mask selects appropriate red, green and blue colors by shadowing selected areas of the screen from electron beams.
Generally, such a shadow mask is made by first coating photoresist on a thin metal plate. The plate is then exposed to light and, subsequently, developed and etched to form a plurality of holes thereon. Thereafter, the plate formed with the holes is annealed by heat treatment under a hydrogen atmosphere at a high temperature. The annealing step removes inner stress from the plate while causing elongation thereof. The plate is then formed with a predetermined mask shape by the use of a press and degreased to remove oil or alien materials therefrom. Finally, for an oxidization-preventing or heat-absorbing purpose, the degreased plate is processed with a blackening step. The resulting mask shaped plate is mounted within the CRT panel to function as a color selecting shadow mask.
The shadow mask acts as a physical barrier to electron beams as they progress from one location to the next, and minimizes the generation of spurious colors by excitation of the wrong phosphor. However, when an external shock or impact or a sound wave from the built-in speaker is applied to the shadow mask, it is liable to be seriously vibrated due to its structural weakness so that the electron beam passing therethrough may land on the wrong phosphor, resulting in a deterioration of color purity.
FIGS. 1 and 2 each show a panel assembly where the aforementioned phenomenon is schematically illustrated. As shown in FIG. 1, the panel assembly includes a panel 1, a phosphor screen 2 formed on an inner surface of the panel 1, and a shadow mask 6 placed directly behind the phosphor screen 2 and provided with a plurality of beam-guide holes 8. The shadow mask 6 is supported by a mask frame 5. The mask frame 5 is coupled to a stud pin 3 protruding from the side wall of the panel 1 by interposing a spring 4 therebetween.
When the external shock or impact is applied to the shadow mask 6, the shadow mask 6 is shaken such that it moves from ts initial position to a new position 7. With the movement of the shadow mask 6, the beam-guide hole 8 also moves from its initial position to a new position 9. Accordingly, an electron beam 10 emitted from an electron gun 11 takes a wrong course such that its landing position is changed from P1 to P2, resulting in excitation of the wrong phosphor.
Furthermore, when a physically strong shock is applied to the CRT, the shadow mask 6 is liable to be partially deformed. Such a deformed portion 12 is indicated in FIG. 2. As the electron beam 10 passes through the deformed portion 12 and lands on the phosphor screen 2, spurious colors are generated.
The aforementioned shortcomings are mainly derived from the poor tensional strength of the shadow mask. Since the shadow mask is usually made of a thin metal plate, it should pass through several rolling steps during the manufacturing process to be provided with appropriate structural hardness. The annealing step is employed to give such a characteristic to the metal plate and is performed before the forming step which requires some degree of elongation of the target material. As shown in FIG. 5, when this annealing step is absent, the metal plate has high tensional strength but poor elongation so that it is practically impossible to form the metal plate with a mask shape. On the contrary, as shown in FIG. 6, when the annealing step is present, the metal plate has high elongation but poor tensional strength so that the resulting shadow mask should bear the aforementioned defects of easily vibrating or deforming at an external shock or impact.
In order to solve the problem, Japanese Patent Laid-Open No. Sho 62-223950 discloses a technique of improving tensional strength of the shadow mask by forming a plating layer thereon. However, this technique involves reduced beam-guide hole size. Furthermore, Japanese Patent Laid-Open Nos. Sho 56-121257 and Hei 1-276542 each disclose a technique of improving tensional strength of the shadow mask by performing gas heat-treatment with respect to the shadow mask. However, with this technique, the shadow mask passing through the press-forming step is heat-treated at a high temperature for a long time so that it may be thermally deformed.
It is an object of the present invention to provide a CRT shadow mask having improved tensional strength and elongation.
It is another object of the present invention to provide a CRT shadow mask having an improved coefficient of elasticity.
It is still another object of the present invention to provide a method of manufacturing a CRT shadow mask having at least one of an improved coeffcient of elasticity, and an improved tensional strength and elongation.
In order to achieve these objects and others, the CRT shadow mask includes a surface hardening layer, and a solid-solution and precipitate hardening layer formed under the surface hardening layer.
The method of manufacturing the CRT shadow mask includes the steps of heat-treating a metal plate with a carbonitriding gas, and forming the metal plate with a predetermined mask shape.